The interim report cited an incident where a patient waited in an ambulance outside A&E for three hours and 20 minutes.
Patients are being put at risk and some are dying due to handover delays between ambulances and A&E departments, safety investigators have said.
A new study from the Healthcare Safety Investigation Branch (HSIB) recommended that the Department of Health and Social Care leads an “immediate strategic national response” to address patient safety issues arising from flow into, through and out of hospitals.
It said there is a link between delays in being able to hand patients over and getting ambulances back on the road to answer new calls – meaning people calling 999 wait longer than they should.
The interim report cited an incident where a patient waited in an ambulance outside A&E for three hours and 20 minutes, under the care of an ambulance crew, before being admitted to intensive care.
The study said: “Hospitals experience pressure due to the number of patients already in hospital beds and people coming into hospital who require emergency admission.
“This creates additional challenges when people who are already in hospital, and who are medically well enough to be discharged to a different place of care, experience delays in getting into community and social care.
“This builds pressure through the entire local healthcare system which is felt and seen by the emergency department and the ambulance services.”
The report said harm was caused by patients waiting for ambulances and waiting in ambulances outside hospitals, which could mean their condition deteriorates, and people in hospital becoming “institutionalised due to unnecessary extended stays”.
The HSIB called on the Department of Health to identify risks to patient safety arising from challenges in constraints, demand, capacity and the flow of patients.
Neil Alexander, lead investigator, said: “As our investigation progresses, we continue to hear about the deep anxiety and distress that delays in handing over care can cause.
“It is one of the most urgent issues facing healthcare, and sadly in the most serious of cases has resulted in deaths of patients who couldn’t access the treatment they needed in time.
“Having heard from both frontline staff and healthcare leaders, it was important to publish what we had found so far from a patient safety perspective and to prompt immediate action.
“Our initial findings point out that each area of the healthcare system is doing what they can to tackle growing pressure.
“Our safety recommendations recognise that this work needs to be brought together under a national, strategically led response that reduces delays and improves patient flow across the health and care system to mitigate the current harm and future risks to patient safety.”
An NHS spokesman said: “Record levels of life-threatening ambulance call-outs and 999 calls and – as this report acknowledges – challenges discharging patients into the community has inevitably had an impact on ambulance waiting times.
“That is why NHS staff continue to work closely with social care colleagues to free up precious bed space for those most in need of care.”